Tungsten-halogen incandescent lamps and arc discharge lamps are well known; see IES LIGHTING HANDBOOK, 1981 Reference Volume, Section 8. There is a small probability that a metal halide or tungsten-halogen lamp will shatter during operation of the lamp. On the infrequent occasion that a lamp shatters, the sequence of events within the lamp is as follows: the inner light-source capsule bursts causing fragments of glass or shards to be propelled against the outer envelope; these shards cause the outer envelope of the lamp to shatter. This type of lamp failure will hereinafter be referred to as a "containment failure" of the lamp.
The causes of these infrequent containment failures are varied and unpredictable. There is no known way to eliminate the possibility of such failures. Although occurrence of the failure is rare, nevertheless it could present a safety hazard to a person in the immediate vicinity of a lamp or a possibility of damage to nearby property. Where such failures can be anticipated, lamp manufacturers notify users by means of warnings on packages and other descriptive materials and by suggested precautions in specifications. This hazard may be avoided by operating the lamp in a fixture designed to contain such a failure. The requirement that the lamp be operated in a protective fixture is frequently employed in commercial usage. However, this procedural safeguard is less acceptable for consumer usage.
The lighting industry is searching for a replacement for the Edison-type incandescent lamp which is currently the most popular type of lamp sold in the consumer market in the United States. Tungsten-halogen and arc discharge lamps, because of their superior performance characteristics, are being carefully considered by various lamp manufacturers as a replacement for the standard incandescent lamp. However, the remote possibility of a containment failure is a substantial impediment in the path of developing a feasible replacement in the consumer market. A tungsten-halogen lamp or an arc discharge lamp which substantially eliminates the risk of a containment failure would constitute an advancement in the art.
Various methods have been suggested to improve the ability of tungsten halogen and arc discharge lamps to withstand a burst of the inner light-source capsule. These methods may attempt to restrict shards from impacting with the outer envelope, may reinforce the outer envelope so that it will not shatter in the event shards are propelled against it, or may employ a combination of both techniques. The practice of applying a light-transmissive coating or covering on the inside or outside surface of the outer envelope as reinforcement thereof is well known in the art. Bechard et al., in U.S. Pat. No. 4,281,274, issued July 28, 1981, disclose an enclosure of glass surrounding the arc tube of an arc discharge lamp as a containment device. In copending U.S. patent application, Ser. No. 422,311, filed on Sept. 23, 1982, and assigned to the assignee hereof, there is disclosed a knitted wire mesh containment device surrounding the light-source capsule of an arc discharge lamp or a tungsten-halogen lamp. A light-source capsule containment device comprising glass wool being packed approximately uniformly between the light-source capsule and the outer envelope is disclosed in copending U.S. patent application, Ser. No. 440,123, filed Nov. 8, 1982, and assigned to the assignee hereof. Abandoned U.S. patent application, Ser. No. 845,738, filed Oct. 26, 1977, by Fridrich, referred to in U.S. Pat. No. 4,151,445, suggests that the risk of a containment failure may substantially be eliminated in an arc discharge lamp having a miniaturized arc tube with thinner walls.
These methods of containment generally necessitate additional hardware and result in increased costs of manufacture. There is usually some loss of efficacy associated with these methods. In most instances, such containment devices detract from the aesthetic appearances of lamps. There is no means of containment available in the existing art for general lighting applications which is reliable, which incures no additional manufacturing costs, which results in no measurable loss of efficacy, and which does not detract from the aesthetic quality of the lighting product.